The earliest communication system by telephone was the Public Switched Telephone Network (PSTN) in which the telephones of two parties were connected by copper wire through switches controlled by a central office.
Although this network was created using analog voice connections through manual switches, automated telephone exchanges replaced most switchboards. Most switches now use digital circuits between exchanges, with analog two-wire circuits still used to connect to most telephones.
Voice over IP (VoIP) technology allows telephone calls to be made over computer networks like the Internet. VoIP converts analog voice signals into digital data packets and supports real-time, two-way transmission of conversations using Internet Protocol (IP).
Providers of VoIP, offer a variety of services. The most common application of VoIP for personal or home use is internet-based phone services that rely on a telephone switch. With this application, you will still have a phone number, will still dial phone numbers, and will likely have an adapter that allows you to use a regular telephone. The person you are calling will not likely notice a difference from a traditional phone call.
VoIP phone service for consumers and small businesses typically runs over broadband equipment Internet or similar broadband connection. The problem is that VOIP may delay data transmission and cause voice jitter This causes malfunction of the modem.
Although the PSTN system was initially the only system in use, the volume of Voice over IP usage has grown dramatically in recent years. Many customers subscribe for service from both a provider and a central office. The typical situation is where a customer initiating or receiving a call does not know whether an incoming call is from VoIP or from PSTN. Therefore when his phone service has a problem such as jitter, he is uncertain as to whether the problem originates between his phone and the central office of PSTN or with a provider of Voice over IP protocol.